1. Field of the Invention
The present invention relates to a method for making a composite foamed and shaped article suitable for use for the interior of cars or surface fitting of furniture.
2. Description of the Related Art
A method for making a composite foamed and shaped article suitable for use for the interior of cars or surface fitting of furniture utilizing the powder slushing method is already known.
For example, a method of making a composite foamed and shaped article, is disclosed in Japanese Examined patent publication ( KOKOKU ) No.42-11510, including a first step in which a thermoplastic resin powder is fied in a heated slush molding die which is then heated so as to form a first resin layer and then the surplus thermoplastic resin powder is discharged; a second step in which a mixed powder consisting of heat decomposable foaming agent and the thermoplastic resin powder is filled in the slush molding die where the first resin layer is formed and then reheated to form a sintered second resin layer on the first resin layer and then the surplus mixed resin powder is discharged and a third step in which the slush molding die in which the first and the second resin layers are formed is further heated to foam the second resin layer and then the die is cooled and the foamed and shaped article is taken out from the die.
In this method, however, the slush molding die is heated to form the sintered resin layer in the first and the second steps, so it takes a relatively long time to obtain a shaped article, which is disadvantageous in productivity.
To overcome this problem in the prior art, another proposal Japanese Examined Patent Publication ( KOKOKU ) No.63-27167 discloses a first step in which a thermoplastic resin powder is filled in a preheated slush molding die to form a first resin layer and a second step in which a mixed powder is filled in the slush molding die and heated to be adhered to the first resin layer, utilizing the remaining heat of the die, to form a second resin layer.
Even utilizing this method, however, the time required to produce a single shaped article, i.e., the molding time, cannot sufficiently be shortened.
That is, in a conventional molding die heating method utilizing heated air or the like, a problem arises in that it is difficult to rapidly and uniformly heat a molding die with heated air.
Therefore, even if the heating of the molding die in the second step is eliminated, the time required to produce a shaped article cannot be sufficiently shortened.
That is, a heating method has never been obtained which can fully meet with these conditions.
The hot air heating method is the most typical heating method.
However, in this method, air is heated and the die is heated by the thus heated air.
Air has an extremely low coefficient of thermal conductivity.
Accordingly, a lot of heat sources are required to heat the heating medium per se, and the heating time is naturally extended.
Further, it is difficult for the die to be heated uniformly.
On the other hand, there is a method in which use is made of oil as a heating medium. Oil is heated by a suitable heating source and supplied to conduits arranged on the outer peripheral surface of the die to heat it.
This method has an advantage over the air heating method in view of the heating medium (the coefficient of thermal conductivity of oil is higher than that of air), but the heating temperature of the die can be easily be affected by the degree of contact between the die and the conduits the position on the outer surface of the die on which the conduits are installed, the number of the conduits installed per unit area of the surface of the die, etc.
Thus, a lot of time and expense are required for manufacturing he die and for heating tests.
Further, the die cannot be heated sufficiently uniformly and rapidly in spite of taking a lot of time and expense.
Moreover, a cooing oil should be prepared to cool the die.
In the induction heating method, induction coils are arranged on the outer peripheral surface of the die and actuated through an oscillator.
Since the induction coils are installed on the outer surface of the die, however this method has the same drawbacks as the oil heating method.